Justice Anthony Kennedy, the court's frequent swing vote, joined more liberal justices in questioning Florida's rigid IQ score threshold for determining intellectual disability. Kennedy's positioning hinted at the possibility that the court might strike down the strict IQ rule used by Florida, Idaho, Kentucky and several other states with the death penalty.

The Supreme Court decided in Atkins vs. Virginia in 2002 that the execution of those variously called mentally retarded or intellectually disabled violates the Eighth Amendment's prohibition against cruel and unusual punishment.

The Atkins decision gave states substantial discretion and only general guidance. It said a finding of intellectual disability requires proof of three things: "subaverage intellectual functioning," meaning low IQ scores; a lack of fundamental social and practical skills; and the presence of both conditions before age 18. The court said IQ scores under "approximately 70" typically indicate intellectual disability.

Florida imposes a three-part test, which starts with a rigid requirement that the inmate score 70 or below on the IQ test. If the inmate scores below the cutoff number, the state also will assess for "deficits in adaptive behavior" and an onset before the age of 18.

Freddie Lee Hall, the 68-year-old convicted murderer whose case was before the Supreme Court on Monday, has been on the state's death row since 1978. Hall was convicted in the murders of a pregnant Sumter County woman and Hernando County sheriff's Deputy Lonnie Coburn. Hall and an accomplice, Mack Ruffin, were given life sentences for Coburn's death.

Hall had generally scored slightly above 70 on IQ tests.

Florida Solicitor General Allen Winsor said the state's approach was "a reasonable legislative judgment," one he said was followed in eight states.

But Kennedy and Justice Elena Kagan said the effect of Florida's rigid approach is to stop consideration of the other two factors in the analysis suggested in the Atkins decision.